1. Technical Field
The present invention generally relates to multiple access control to a communication link, such as the uplink in a wireless communication network, and particularly relates to using a hybrid of contention-based access and schedule-based access.
2. Background
As current and future wireless communication networks transition away from circuit-switched communications and toward packet-switched communications, resource allocation becomes more flexible and more challenging. For example, in packet-switched environments, resource allocations tend to be made on a demand basis. Further, resource allocations to individual users tend to be of short duration, reflecting the bursty, intermittent nature of many packet-switched communication applications, such as web browsing, etc.
As one example, packet-based communication applications and services complicate the allocation of common uplink resources in cellular and other wireless communication networks, such as where a potentially large group of users in a given cell share a common communication uplink. One sharing approach depends on resource reservation techniques, wherein individual users access the common uplink on a reserved basis. Scheduling access to the communication link in this manner has the advantage of controlling the group of users in a manner that avoids conflicting transmissions (i.e. simultaneous transmission on the same physical resource by more than one user) but the scheduling request/grant signaling adds potentially significant overhead to the communication link, and adds complexity to the managing base station.
As an alternative, contention-based access offers potentially greater communication link efficiency by eliminating the signaling overhead of scheduled access. With contention-based access, each user vying for access to the communication link transmits autonomously, meaning that collisions between user transmissions occur from time to time. Commonly, the base station or other receiving entity can demodulate more than one user's transmission at the same time, so collisions do not necessarily mean loss of user data. However, interference or practical constraints on demodulation resources generally limits the number of simultaneous user transmissions that can be successfully received. The term “contention overload” denotes the occurrence of more than the maximum allowed number of simultaneous user transmissions, where that number may be fixed, or may change depending upon prevailing reception conditions and loading at the base station.
As a common point, conventional contention-based access schemes assume that no user transmissions can be received (demodulated) correctly in the presence of a contention overload, meaning that all user transmissions are “lost” in such events and the channel resources used for those transmissions were completely wasted. Worse, without provisions to stagger or otherwise disperse retransmissions on the contention-based channel, the users involved in a contention overload all may attempt retransmissions at or about the same time, thereby risking a further contention overload. As a general proposition, therefore, contention overloads may be understood as potentially reducing or eliminating the efficiency gains that come with the elimination of scheduling overhead from the communication link.